A fuel cell is an electrochemical conversion device that directly converts chemical energy into electric energy. This conversion is accomplished by supplying a fuel and an oxidizing agent to two electrodes that are electrically connected and electrochemically inducing oxidation of the fuel. A fuel cell may be configured by stacking a plurality of unit cells containing, as a basic structure, a membrane-electrode assembly (MEA) in which an electrolyte membrane is sandwiched between a pair of electrodes. Among such fuel cells, a proton exchange membrane fuel cell (PEMFC), which uses a solid polymer electrolyte membrane as the electrolyte membrane, is a particularly attractive power source.
A proton exchange membrane fuel cell (PEMFC) transforms the chemical energy liberated during the electrochemical reaction of hydrogen and oxygen to electrical energy, as opposed to the direct combustion of hydrogen and oxygen gases to produce thermal energy. A stream of hydrogen is delivered to the anode side of the membrane electrode assembly (MEA) and is catalytically split into protons and electrons. This oxidation half-cell reaction is represented in Equation 1:H2→2H++2e− E°=0VSHE   (Equation 1)
The newly formed protons permeate through the polymer electrolyte membrane to the cathode side. The electrons travel along an external load circuit to the cathode side of the MEA, thus creating the current output of the fuel cell.
Meanwhile, a stream of oxygen is delivered to the cathode side of the MEA. At the cathode side oxygen molecules react with the protons permeating through the polymer electrolyte membrane and the electrons arriving through the external circuit to form water molecules. This reduction half-cell reaction is represented in Equation 2:O2+4 H++4e−→2 H2O E°=1.229VSHE   (Equation 2)
The proton conducting polymer membrane, which primarily comprises a proton conducting polymer, is a central component of PEMFCs. The proton conducting polymer, which is a species of an ionomer, is sandwiched between anode and cathode. The primary function of these membranes is to carry protons from the anode to the cathode.
Sulfonated tetrafluoroethylene based fluoropolymer-copolymers, such as Nafion®, sulfonated poly(ether ether ketone) such as SPEEK, sulfonated polyimides and others proton conducting polymers are generally used in PEMFCs and DMFCs. Nafion® is generally considered the ionomer of choice in the catalyst matrix to transport protons to and from bulk electrolyte to the catalyst matrix. Proton exchange membrane polymers, especially Nafion®, perform best in the temperature range of about 65° C. to about 80° C. in fuel cells. At higher temperatures, these polymers display function deterioration due to poor proton conductivity, dehydration, and lack of thermal and mechanical stability. Anion exchange membranes (AEM), such as quaternary ammonium cation functionalized polymers, can display similar function deterioration and encounter stability issues with increasing temperature.
On the other hand, platinum, which is the universally-accepted electro-catalyst for both anodes and cathodes, is very expensive and performs better at higher temperatures, especially for oxygen reduction. Other electro-catalysts, such as non-platinum metals or very low platinum-content metal alloys, which may be more cost effective than pure platinum, tend to perform adequately at temperatures higher than 100° C. These higher temperatures tend to reduce catalyst deactivation by carbon monoxide poisoning. But high performance of precious and non-precious catalysts is difficult to exploit due to limitations inherent to the proton exchange membrane polymers, as discussed above.
Attempts have been made to increase the performance, (e.g., thermal, chemical and mechanical stability) of conducting polymers and ionomers by incorporating oxide fillers, such as silica, in the polymer matrix. However, polymers which incorporate oxide fillers have encountered problems, such as stability over time due to phase separation. Moreover, the preparation of the oxide incorporated fillers is a multi-step time consuming process. Therefore, a need exists for conducting polymers with increased stability and methods of making the same.